Patent Document 1 indicates that a fluorine-containing elastomer comprising vinylidene fluoride units [VdF], hexafluoropropylene units [HFP], and optionally tetrafluoroethylene units [TFE], and having a specific multi-peak type molecular weight distribution and a specific molecular weight exhibits excellent heat resistance, oil resistance, and chemical resistance, undergoes less change in physical properties caused by roll kneading, and has excellent extrudability.
In order to solve the problem of variations in extrusion rate and physical properties in a method described in Patent Document 2 that satisfies both physical properties and extrudability by means of a multi-peak type molecular weight distribution having two or more peaks, Patent Document 1 proposes a fluorine-containing elastomer having the following features:                (a) the molecular weight distribution is a multi-peak type;        (b) the molecular weight at the peak top of the highest molecular weight is 350,000 to 550,000;        (c) the Mw/Mn ratio is 4 to 10;        (d) the intrinsic viscosity is 40 to 200 ml/g; and        (e) the ratio of the content of the component with a molecular weight of 50,000 or less to the intrinsic viscosity number is 0.15 to 0.70.        
In Patent Documents 1 and 2, two fluorine-containing elastomers having different molecular weight distributions, which are obtained by a suspension polymerization method, are blended to achieve a multi-peak type molecular weight distribution. Patent Documents 1 and 2 teach that an iodine-containing compound, preferably such as diiodomethane, is used in this case to particularly control the molecular weight of the lower-molecular-weight fluorine-containing elastomer. These patent documents are reportedly intended to improve extrudability; however, as for specific applications, they merely refer to automobile fuel hoses as a typical application.
Patent Document 3 discloses a low-hardness vulcanizable fluororubber composition comprising a fluorine-containing elastomer (not a blend) comprising a copolymer of VdF and HFP or a terpolymer of VdF, HFP, and 35 wt. % or less of TFE, a crosslinking agent, a vulcanization accelerator, and an oxide or hydroxide of a divalent metal; wherein the fluorine-containing elastomer has an intrinsic viscosity number of 50 to 200 ml/g, and has a double-peak type molecular weight distribution in which the molecular weight distribution of the components with a molecular weight of 50,000 or less is 20 to 70%, and with a molecular weight of 1,000,000 or more is 2 to 30%. This composition achieves hardness as low as a JIS-A hardness (spring type hardness) of 50 or less, which is considered to be suitable for use in vibration-absorbing rubber and in seal parts to which sealing properties are required with a low clamping force.
In addition, Patent Document 4, which describes an invention filed by the present applicant, discloses a fluorine-containing elastomer composition comprising a fluorine-containing elastomer for molding fuel system parts, at least one of a polyol-vulcanizing agent and a polyamine-based vulcanizing agent, an organic peroxide, and a polyfunctional unsaturated compound; wherein the fluorine-containing elastomer is copolymerized in the presence of an iodine- and bromine-containing compound represented by the general formula: RBrnIm, having a copolymerization composition comprising 10 to 40 mol % of TFE, 80 to 30 mol % of VdF, and 10 to 30 mol % of HFP, an intrinsic viscosity [η] of 20 to 180 ml/g, an Mw/Mn ratio of 2 to 20, and a mono-peak type or multi-peak type molecular weight distribution.
Such a preferable multi-peak type molecular weight distribution of the fluorine-containing elastomer has two or more peaks of the low-molecular-weight component, which imparts processability, and the high-molecular-weight component, which imparts physical properties. An elastomer containing iodine and bromine groups derived from an iodine- and bromine-containing compound is at least used as the low-molecular-weight component. Actually, a high-molecular-weight polymer and a low-molecular-weight polymer each prepared by an emulsion polymerization method are blended by, for example, mixing latexes of these elastomers in a desired proportion, stirring the resulting mixture, and then adding a saline solution, etc., to perform coagulation.
The thus-obtained fluorine-containing elastomer with a multi-peak type molecular weight distribution is suitably used for molding fuel system parts; however, when the elastomer is used for sealing material applications, a further improvement in hardness is required.